Fuel cell system

ABSTRACT

A fuel cell system includes: a first fuel cell; a second fuel cell having a greater maximum power output than a maximum power output of the first fuel cell; and a controller configured to cause the first fuel cell to generate greater electric power greater than the second fuel cell when the requested power is smaller than a first threshold, cause the second fuel cell to generate greater electric power than the first fuel cell when the requested power is a second threshold, which is the first threshold or greater, or greater and is smaller than a third threshold that is greater than the second threshold and is greater than 50% of a sum of the maximum power outputs of the first and second fuel cells, and cause both the first and second fuel cells to generate electric power when the requested power is the third threshold or greater.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2018-157673, filed on Aug. 24,2018, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a fuel cell system.

BACKGROUND

Fuel cell systems each including a plurality of fuel cells are known asconventional fuel cell systems. For example, there is a known fuel cellsystem that includes: first and second fuel cells that differ in powergeneration efficiency, to generate electric power constantly at highefficiency; and a switch control unit that switches between the firstand second fuel cells that supply electric power to a power receivingunit, to maximize power generation efficiency in response to an outputrequest from the power receiving unit as disclosed in, for example,Japanese Patent Laid-Open No. 2016-091625 (hereinafter, referred to asPatent Document 1).

SUMMARY

It is therefore an object of the present disclosure improve durability.

The above object is achieved by a fuel cell system including: a firstfuel cell; a second fuel cell having a maximum power output that isgreater than a maximum power output of the first fuel cell; and a powergeneration controller configured to control power generation from thefirst fuel cell and the second fuel cell in accordance with requestedpower, wherein, when the requested power is smaller than a firstthreshold, the power generation controller is configured to cause thefirst fuel cell to generate electric power greater than electric powerof the second fuel cell so that the requested power is supplied, whenthe requested power is equal to or greater than a second threshold thatis equal to or greater than the first threshold, and is smaller than athird threshold that is greater than the second threshold and is greaterthan 50% of a sum of the maximum power output of the first fuel cell andthe maximum power output of the second fuel cell, the power generationcontroller is configured to cause the second fuel cell to generateelectric power greater than electric power of the first fuel cell sothat the requested power is supplied, and when the requested power isequal to or greater than the third threshold, the power generationcontroller is configured to cause both the first fuel cell and thesecond fuel cell to generate electric power so that the requested poweris supplied.

In the above configuration, the power generation controller may beconfigured to suspend power generation from the second fuel cell whenthe requested power is smaller than the first threshold, and/or may beconfigured to suspend power generation from the first fuel cell when therequested power is equal to or greater than the second threshold and issmaller than the third threshold.

In the above configuration, the power generation controller may beconfigured to cause the second fuel cell to generate electric power at avoltage at which elution of a catalyst contained in the second fuel cellis inhibited when the requested power is smaller than the firstthreshold, and/or may be configured to cause the first fuel cell togenerate electric power at a voltage at which elution of a catalystcontained in the first fuel cell is inhibited when the requested poweris equal to or greater than the second threshold and is smaller than thethird threshold.

In the above configuration, the second threshold may have a greatervalue than the first threshold, and, when the requested power is equalto or greater than the first threshold and is smaller than the secondthreshold, the power generation controller may be configured to causethe first fuel cell and the second fuel cell to generate electric powerso that the requested power is provided by both the first fuel cell andthe second fuel cell.

In the above configuration, when the requested power is equal to orgreater than the first threshold and is smaller than the secondthreshold, the power generation controller may be configured to causeoutput power of the first fuel cell to become smaller and may beconfigured to cause output power of the second fuel cell to becomeslarger in response to an increase in the requested power.

In the above configuration, the second threshold may have the same valueas the first threshold.

In the above configuration, when a rate of change in the requested poweris equal to or higher than a predetermined value, the power generationcontroller may be configured to reduce at least one of the firstthreshold and the third threshold to a smaller value than in a casewhere the rate of change in the requested power is lower than thepredetermined value.

In the above configuration, the fuel cell system may further include astorage unit that stores a map showing a correlation between requestedpower and an operation time of each of the first fuel cell and thesecond fuel cell, wherein, on the basis of the map stored in the storageunit, the power generation controller may be configured to change atleast one of the first threshold and the third threshold so that theoperation time of the second fuel cell becomes equal to or greater than80% of the operation time of the first fuel cell and equal to or lessthan 120% of the operation time of the first fuel cell.

In the above configuration, on the basis of a map received from anexternal server and showing a correlation between requested power and anoperation time in another fuel cell system, the power generationcontroller may be configured to change at least one of the firstthreshold and the third threshold so that an operation time of thesecond fuel cell becomes equal to or greater than 80% of an operationtime of the first fuel cell and equal to or less than 120% of theoperation time of the first fuel cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the configuration of a fuel cellsystem according to a first embodiment;

FIG. 2 is a current-power characteristics chart showing the relationshipbetween output currents and output powers of a first fuel cell and asecond fuel cell;

FIG. 3 is a chart for explaining the maximum power output in a casewhere a maximum allowable current and a minimum allowable voltage areset in the second fuel cell;

FIG. 4 is a schematic diagram showing the electrical configuration ofthe fuel cell system according to the first embodiment;

FIG. 5 is a flowchart showing a power generation control process in thefirst embodiment;

FIG. 6 is a timing chart for explaining power generation control in thefirst embodiment;

FIG. 7 is a chart for explaining power generation control in the firstembodiment;

FIG. 8 is a chart for explaining power generation control in acomparative example;

FIG. 9 is a flowchart showing a power generation control process in asecond embodiment;

FIG. 10 is a timing chart for explaining power generation control in thesecond embodiment;

FIG. 11 is a chart for explaining power generation control in the secondembodiment;

FIG. 12 is a flowchart showing a power generation control process in athird embodiment;

FIG. 13 is a timing chart for explaining power generation control in thethird embodiment;

FIG. 14 is a schematic diagram showing the configuration of a fuel cellsystem according to a fourth embodiment; and

FIG. 15 is a chart showing an example of an operation history stored ina storage unit.

DETAILED DESCRIPTION

The fuel cell system disclosed in Patent Document 1 aims to increasepower generation efficiency for requested power, and there is a marginfor improvement in increasing durability of the fuel cell system.

The following is a description of embodiments of the present disclosure,with reference to the accompanying drawings.

First Embodiment

FIG. 1 is a schematic diagram showing the configuration of a fuel cellsystem according to a first embodiment. A fuel cell system is a powergenerating system that is used in a fuel cell vehicle or a stationaryfuel cell device or the like, and outputs electric power in accordancewith requested power. Note that, in the example cases in the embodimentsdescribed below, a fuel cell system is mounted in a vehicle. As shown inFIG. 1, a fuel cell system 100 includes a first fuel cell 10, a secondfuel cell 11, a control unit 20, cathode gas piping systems 30 and 40,and anode gas piping systems 50 and 70. Note that the fuel cell system100 also includes a refrigerant piping system, but the refrigerantpiping system is neither shown nor explained herein.

The first fuel cell 10 and the second fuel cell 11 are solid polymerelectrolyte fuel cells that generate electric power upon receipt of asupply of hydrogen (anode gas) and air (cathode gas) as reaction gases.The first fuel cell 10 and the second fuel cell 11 each have a stackstructure in which a plurality of cells are stacked. Each of the cellsincludes a membrane electrode joined member that is a power generatingmember having electrodes disposed on both sides of an electrolytemembrane, and a pair of separators between which the membrane electrodejoined member is interposed.

The electrolyte membrane is a solid polymer membrane formed with afluorine-based resin material or a hydrocarbon-based resin materialcontaining sulfonate groups, and has excellent protonic conductivity ina wet state. The electrodes contain carbon carriers. The carbon carriersin the electrodes carry the catalyst (platinum, or a platinum-cobaltalloy) for accelerating a power generating reaction. A manifold forsupplying the reaction gases is provided in each cell. The reactiongases flowing in the manifolds are supplied to the power generatingregions in the respective cells through gas flow channels formed in therespective cells.

The maximum power output of the second fuel cell 11 is higher than thatof the first fuel cell 10. For example, the number of stacked cells inthe second fuel cell 11 is larger than that in the first fuel cell 10,and accordingly, the second fuel cell 11 has the higher maximum poweroutput. The maximum power output of the second fuel cell 11 may be atleast 1.5 times, 1.8 times, or 2.0 times higher than the maximum poweroutput of the first fuel cell 10. The maximum power output of the secondfurl cell 11 may be not higher than 3.0 times, 2.8 times, or 2.5 timesthe maximum power output of the first fuel cell 10.

FIG. 2 is a current-power characteristics chart showing the relationshipbetween the output current and the output power of each of the firstfuel cell and the second fuel cell. As shown in FIG. 2, the maximumpower output P2 of the second fuel cell 11 is higher than the maximumpower output P1 of the first fuel cell 10. Further, the first fuel cell10 and the second fuel cell 11 differ in the number of cells that areformed with the same material and have the same power generating area,and accordingly, differ in maximum power output. Because of this, theoutput current with which the first fuel cell 10 reaches the maximumpower output P1, and the output current with which the second fuel cell11 reaches the maximum power output P2 fall in the neighborhood of anoutput current A (or are the same output current A). Note that thesecond fuel cell 11 may have the same number of stacked cells as that inthe first fuel cell 10, but differ in the material and/or the powergenerating area of the cells from the first fuel cell 10, to have thehigher maximum power output.

In a case where a maximum allowable current and/or a minimum allowablevoltage is set for the output current and/or the output voltage of eachof the first fuel cell 10 and the second fuel cell 11 to avoid a rapidvoltage drop or reduce heat generation from the fuel cells, the maximumpower output of each of the first fuel cell 10 and the second fuel cell11 may be an output power that becomes highest within the allowablerange.

FIG. 3 is a chart for explaining the maximum power output in a casewhere a maximum allowable current and a minimum allowable voltage areset in the second fuel cell. As shown in FIG. 3, in a case where amaximum allowable current LA and/or a minimum allowable voltage LV isset, the maximum power output P2 a within the allowable current rangeand/or the allowable voltage range may be the maximum power output ofthe second fuel cell 11. The same applies to the first fuel cell 10.

As shown in FIG. 1, the control unit 20 receives an accelerator positionsignal transmitted from an accelerator pedal sensor 67 that detects theposition of an accelerator pedal 66 (or the pressure applied onto theaccelerator pedal 66 by the driver). The control unit 20 calculates therequested power from the accelerator position signal, and controls thelater described components of the fuel cell system 100 in accordancewith the calculated requested power, to control power generation fromthe first fuel cell 10 and the second fuel cell 11. In other words, thecontrol unit 20 functions as a power generation controller 22 thatcontrols power generation from the first fuel cell 10 and the secondfuel cell 11. Here, the electric power requested from the entire fuelcell system 100 including the first fuel cell 10 and the second fuelcell 11 is first calculated in accordance with the accelerator position.In a case where the fuel cell system 100 includes a secondary cell, thecharging status of the secondary cell may be detected, and the electricpowers requested from the first fuel cell 10 and the second fuel cell 11may be calculated while the electric power being stored into andreleased from the secondary cell is taken into account.

The cathode gas piping system 30 supplies the cathode gas to the firstfuel cell 10, and releases a cathode exhaust gas that is the gas notconsumed in the first fuel cell 10. The cathode gas piping system 30includes a cathode gas pipe 31, an air compressor 32, an open/closevalve 33, a cathode exhaust gas pipe 34, and a pressure adjusting valve35. The cathode gas pipe 31 is a pipe connected to a cathode inlet ofthe first fuel cell 10. The air compressor 32 is connected to thecathode of the first fuel cell 10 via the cathode gas pipe 31, andsupplies air that is taken from outside and compressed, as the cathodegas to the first fuel cell 10. The control unit 20 controls driving ofthe air compressor 32, to control the amount of the air to be suppliedto the first fuel cell 10. The open/close valve 33 is disposed betweenthe air compressor 32 and the first fuel cell 10, and opens/closes inaccordance with the flow of the air in the cathode gas pipe 31. Forexample, the open/close valve 33 is normally in a closed state, andopens when air at a predetermined pressure is supplied from the aircompressor 32 to the cathode gas pipe 31. The cathode exhaust gas pipe34 is a pipe connected to a cathode outlet of the first fuel cell 10,and releases the cathode exhaust gas to the outside of the fuel cellsystem 100. The pressure adjusting valve 35 adjusts the pressure on thecathode exhaust gas in the cathode exhaust gas pipe 34.

The cathode gas piping system 40 supplies the cathode gas to the secondfuel cell 11, and releases a cathode exhaust gas that is the gas notconsumed in the second fuel cell 11. The cathode gas piping system 40includes a cathode gas pipe 41, an air compressor 42, an open/closevalve 43, a cathode exhaust gas pipe 44, and a pressure adjusting valve45. The cathode gas pipe 41, the air compressor 42, the open/close valve43, the cathode exhaust gas pipe 44, and the pressure adjusting valve 45have the same functions as those of the cathode gas pipe 31, the aircompressor 32, the open/close valve 33, the cathode exhaust gas pipe 34,and the pressure adjusting valve 35 of the cathode gas piping system 30.Accordingly, the control unit 20 controls driving of the air compressor42, to control the amount of the air to be supplied to the second fuelcell 11.

The anode gas piping system 50 supplies the anode gas to the first fuelcell 10, and releases an anode exhaust gas that is the gas not consumedin the first fuel cell 10. The anode gas piping system 50 includes ananode gas pipe 51, an open/close valve 52, a regulator 53, an injector54, an anode exhaust gas pipe 55, a gas-liquid separator 56, an anodegas circulating pipe 57, a circulating pump 58, an anode drain pipe 59,and a drain valve 60. The anode gas pipe 51 is a pipe that connects ahydrogen tank 65 and an anode inlet of the first fuel cell 10. In otherwords, the hydrogen tank 65 is connected to the anode of the first fuelcell 10 via the anode gas pipe 51, and supplies hydrogen stored in thetank to the first fuel cell 10. The open/close valve 52, the regulator53, and the injector 54 are disposed in the anode gas pipe 51 in thisorder from the upstream side. The open/close valve 52 opens/closes inaccordance with an instruction from the control unit 20, to control theflow of hydrogen from the hydrogen tank 65 to the upstream side of theinjector 54. The regulator 53 is a pressure reducing valve for adjustingthe pressure on the hydrogen on the upstream side of the injector 54.The injector 54 is an electromagnetically-driven open/close valve thatis electromagnetically driven by a valve unit in accordance with a drivecycle and a valve opening time that are set by the control unit 20. Thecontrol unit 20 controls the drive cycle and/or the valve opening timeof the injector 54, to control the amount of the hydrogen to be suppliedto the first fuel cell 10.

The anode exhaust gas pipe 55 is a pipe that connects an anode outlet ofthe first fuel cell 10 and the gas-liquid separator 56, and introducesan anode exhaust gas containing an unreacted gas (such as hydrogen andnitrogen) not used in any power generating reaction into the gas-liquidseparator 56. The gas-liquid separator 56 separates the gas componentand the water contained in the anode exhaust gas from each other. Thegas-liquid separator 56 then introduces the gas component into the anodegas circulating pipe 57, and introduces the water into the anode drainpipe 59. The anode gas circulating pipe 57 is connected to the anode gaspipe 51 on the downstream side of the injector 54. The circulating pump58 is disposed in the anode gas circulating pipe 57. The hydrogencontained in the gas component separated by the gas-liquid separator 56is sent into the anode gas pipe 51 by the circulating pump 58. Thecirculating pump 58 is driven in accordance with an instruction from thecontrol unit 20. The anode drain pipe 59 is a pipe for releasing thewater separated by the gas-liquid separator 56 to the outside of thefuel cell system 100. The drain valve 60 is disposed in the anode drainpipe 59, and opens/closes in accordance with an instruction from thecontrol unit 20.

The anode gas piping system 70 supplies the anode gas to the second fuelcell 11, and releases an anode exhaust gas that is the gas not consumedin the second fuel cell 11. The anode gas piping system 70 includes ananode gas pipe 71, an open/close valve 72, a regulator 73, an injector74, an anode exhaust gas pipe 75, a gas-liquid separator 76, an anodegas circulating pipe 77, a circulating pump 78, an anode drain pipe 79,and a drain valve 80. The anode gas pipe 71, the open/close valve 72,the regulator 73, the injector 74, the anode exhaust gas pipe 75, thegas-liquid separator 76, the anode gas circulating pipe 77, thecirculating pump 78, the anode drain pipe 79, and the drain valve 80have the same functions as those of the anode gas pipe 51, theopen/close valve 52, the regulator 53, the injector 54, the anodeexhaust gas pipe 55, the gas-liquid separator 56, the anode gascirculating pipe 57, the circulating pump 58, the anode drain pipe 59,and the drain valve 60 of the anode gas piping system 50. Accordingly,the control unit 20 controls the drive cycle and/or the valve openingtime of the injector 74, to control the amount of the hydrogen to besupplied to the second fuel cell 11.

FIG. 4 is a schematic diagram showing the electrical configuration ofthe fuel cell system according to the first embodiment. The fuel cellsystem 100 includes FDCs 81 a and 81 b, an inverter 82, a motorgenerator 83, a BDC 84, a battery 85, and switches 86 a and 86 b, inaddition to the control unit 20 and the others described above.

The FDCs 81 a and 81 b are DC-DC converters. The FDC 81 a transforms theoutput voltage of the first fuel cell 10, and supplies the result to theinverter 82 and the BDC 84. The FDC 81 b transforms the output voltageof the second fuel cell 11, and supplies the result to the inverter 82and the BDC 84. The BDC 84 is a DC-DC converter. The battery 85 is asecondary cell capable of charging and discharging. The BDC 84 canadjust a DC voltage from the battery 85 and output the result to theinverter 82, and can adjust DC voltages from the first fuel cell 10 andthe second fuel cell 11 and a voltage that has been output from themotor generator 83 and converted into a DC voltage by the inverter 82,and outputs the results to the battery 85. The inverter 82 is a DC-ADinverter. The inverter 82 converts DC powers output from the first andsecond fuel cells 10 and 11 and the battery 85, and supplies the resultsto the motor generator 83. The motor generator 83 drives wheels 68. Theswitches 86 a and 86 b open/close in accordance with an instruction fromthe control unit 20, to switch between electrical connection anddisconnection between the first and second fuel cells 10 and 11 and themotor generator 83.

The control unit 20 includes a microcomputer that includes a centralprocessing unit (CPU), a random access unit (RAM), a read only memory(ROM), and a storage unit. The storage unit is a nonvolatile memory suchas a hard disk drive (HDD) or a flash memory, for example. The controlunit 20 comprehensively controls the respective components of the fuelcell system 100, to control operation of the fuel cell system 100. Forexample, the control unit 20 acquires an accelerator position signalfrom the accelerator pedal sensor 67 that detects a position of theaccelerator pedal 66, and calculates the requested power from theacquired accelerator position signal. The control unit 20 then functionsas the power generation controller 22 that controls the amounts of thegases to be supplied to the first fuel cell 10 and the second fuel cell11, the duty ratios of the FDCs 81 a and 81 b, and the like inaccordance with the requested power, to control power generation fromthe first fuel cell 10 and the second fuel cell 11. For example thepower generation controller 22 controls the air compressors 32 and 42,for example, to control the amount of the cathode gas to be supplied tothe first fuel cell 10 and the second fuel cell 11. The power generationcontroller 22 controls the injectors 54 and 74, the circulating pumps 58and 78, and the like, to control the amount of the anode gas to besupplied to the first fuel cell 10 and the second fuel cell 11.

FIG. 5 is a flowchart showing a power generation control process in thefirst embodiment. FIG. 6 is a timing chart for explaining powergeneration control in the first embodiment. As shown in FIG. 5, thecontrol unit 20 stands by until acquiring an accelerator position signaltransmitted from the accelerator pedal sensor 67 (step S10). Afteracquiring an accelerator position signal (step S10: Yes), the controlunit 20 calculates the requested power in accordance with theaccelerator position signal (step S12). For example, the control unit 20refers to a map that is stored in the storage unit and shows therelationship between the accelerator position signal and requestedpower, to calculate the requested power from the acquired acceleratorposition signal.

The control unit 20 then determines whether the calculated requestedpower is smaller than a first threshold (step S14). The first thresholdmay be a value that is not smaller than 70% and not greater than 100% ofthe maximum power output of the first fuel cell 10, for example. Thefirst threshold is stored in the storage unit of the control unit 20,for example. Note that the first threshold may be determined from themaximum power output of the first fuel cell 10 in the initial state, ormay be determined from the maximum power output of the first fuel cell10 acquired at a predetermined time, because the maximum power output ofthe first fuel cell 10 decreases depending on usage conditions.

In a case where the control unit 20 determines the requested power to besmaller than the first threshold in step S14 (step S14: Yes), thecontrol unit 20 controls the respective components of the fuel cellsystem 100, to cause the first fuel cell 10 to generate electric powerso that the requested power is provided by the first fuel cell 10, andsuspend power generation from the second fuel cell 11 (step S16).Specifically, the control unit 20 controls the air compressor 32, theinjector 54, and the like, so that necessary amounts of air and hydrogenfor power generation to provide the requested power are supplied to thefirst fuel cell 10. The control unit 20 also stops the driving of theair compressor 42, the injector 74, and the like, so that air andhydrogen are not supplied to the second fuel cell 11. Thus, as shown inFIG. 6, the requested power is provided by power generation from thefirst fuel cell 10 before time 1 until which the requested power issmaller than the first threshold. Note that, in this case, the controlunit 20 electrically connects the first fuel cell 10 and the motorgenerator 83 by turning on the switch 86 a, and cuts off the electricalconnection between the second fuel cell 11 and the motor generator 83 byturning off the switch 86 b. As the switch 86 b is turned off, and theelectrical connection between the second fuel cell 11 and the motorgenerator 83 is cut off, power generation from the second fuel cell 11can be suspended even in a situation where the reaction gases are beingsupplied to the second fuel cell 11.

In a case where the control unit 20 determines the requested power notto be smaller than the first threshold in step S14 (step S14: No), thecontrol unit 20 determines whether the requested power is equal to orgreater than the first threshold and is smaller than a second thresholdthat is greater than the first threshold (step S18). The secondthreshold is stored in the storage unit of the control unit 20, forexample.

In a case where the control unit 20 determines that the requested poweris equal to or greater than the first threshold and is smaller than thesecond threshold in step S18 (step S18: Yes), the control unit 20controls the respective components of the fuel cell system 100, andcauses both the first fuel cell 10 and the second fuel cell 11 togenerate electric power to provide the requested power (step S20). Inother words, the control unit 20 controls the air compressor 32, theinjector 54, and the like, so that air and hydrogen are supplied to thefirst fuel cell 10. The control unit 20 also drives the air compressor42, the injector 74, and the like, so that air and hydrogen are suppliedto the second fuel cell 11. With this, during the period from time 1 totime 2, during which the requested power is equal to or greater than thefirst threshold and is smaller than the second threshold, as shown inFIG. 6, the requested power is provided by both the power generationfrom the first fuel cell 10 and the power generation from the secondfuel cell 11. Note that, in this case, the control unit 20 turns on theswitches 86 a and 86 b, to electrically connect the first and secondfuel cells 10 and 11 to the motor generator 83.

In this case, the control unit 20 preferably controls driving of the aircompressor 32, the injector 54, and the like, so that the output powerof the first fuel cell 10 decreases in response to the increase in therequested power. Further, the control unit 20 preferably controls theair compressor 42, the injector 74, and the like, so that the outputpower of the second fuel cell 11 increases in response to the increasein the requested power. The reason that both the first fuel cell 10 andthe second fuel cell 11 are made to generate electric power as describedabove in a case where the requested power is equal to and greater thanthe first threshold and is smaller than the second threshold will bedescribed later.

In a case where the control unit 20 determines the requested power notto be equal to or greater than the first threshold and not to be smallerthan the second threshold in step S18 (step S18: No), the control unit20 determines whether the requested power is equal to or greater thanthe second threshold and is smaller than a third threshold that is avalue greater than 50% of the sum of the maximum power output of thefirst fuel cell 10 and the maximum power output of the second fuel cell11 (step S22). The third threshold is stored in the storage unit of thecontrol unit 20, for example. Note that the third threshold may bedetermined from the maximum power outputs of the first fuel cell 10 andthe second fuel cell 11 in the initial state, or may be determined fromthe maximum power outputs of the first fuel cell 10 and the second fuelcell 11 acquired at a predetermined time, because the maximum poweroutputs of the first fuel cell 10 and the second fuel cell 11 decreasedepending on usage conditions.

In a case where the control unit 20 determines that the requested poweris equal to or greater than the second threshold and is smaller than thethird threshold in step S22 (step S22: Yes), the control unit 20controls the respective components of the fuel cell system 100, tosuspend the power generation from the first fuel cell 10, and cause thesecond fuel cell 11 to generate electric power so that the requestedpower is provided by the second fuel cell 11 (step S24). In other words,the control unit 20 stops the driving of the air compressor 32, theinjector 54, and the like, so that air and hydrogen are not supplied tothe first fuel cell 10. The control unit 20 also drives the aircompressor 42, the injector 74, and the like, so that the necessaryamounts of air and hydrogen for power generation to provide therequested power are supplied to the second fuel cell 11. As a result,during the period from time 2 to time 3, during which the requestedpower is equal to or greater than the second threshold and is smallerthan the third threshold, as shown in FIG. 6, the requested power isprovided by power generation from the second fuel cell 11. Note that, inthis case, the control unit 20 turns on the switch 86 b to electricallyconnect the second fuel cell 11 and the motor generator 83, and turnsoff the switch 86 a to cut off the electrical connection between thefirst fuel cell 10 and the motor generator 83. As the switch 86 a isturned off to cut off the electrical connection between the first fuelcell 10 and the motor generator 83, the power generation from the firstfuel cell 10 can be suspended even in a situation where the reactiongases are being supplied to the first fuel cell 10.

As described above, in a case where the requested power is smaller thanthe first threshold, the requested power is provided primarily by powergeneration from the first fuel cell 10. In a case where the requestedpower is equal to or greater than the second threshold, which is greaterthan the first threshold, and is smaller than the third threshold, whichis greater than the second threshold, the requested power is providedprimarily by power generation from the second fuel cell 11. For example,in a case where the second threshold has the same value as the firstthreshold, when the requested power increases to the first threshold (orthe second threshold), the power generation from the first fuel cell 10is suspended, and the second fuel cell 11 is made to generate electricpower, to provide the requested power. However, there are cases where itis difficult to rapidly increase the power generation amount of thesecond fuel cell 11 to the requested power amount. Likewise, when therequested power decreases to the second threshold (or the firstthreshold), the power generation from the second fuel cell 11 issuspended, and the first fuel cell 10 is made to generate electricpower, to provide the requested power. However, there are cases where itis difficult to rapidly increase the power generation amount of thefirst fuel cell 10 to the requested power amount. This is because it isdifficult to rapidly increase the amount of the air to be supplied fromthe air compressors 32 and 42 to the first fuel cell 10 and the secondfuel cell 11, for example. For this reason, as in steps S18 and S20 inFIG. 5, in a case where the requested power is equal to or greater thanthe first threshold and is smaller than the second threshold, which isgreater than the first threshold, the reaction gases are supplied toboth the first fuel cell 10 and the second fuel cell 11, to cause boththe first fuel cell 10 and the second fuel cell 11 to generate electricpower. With this, it is possible to rapidly adjust the power generationamount of the second fuel cell 11 to the requested power amount when therequested power increases to the second threshold, and it is possible torapidly adjust the power generation amount of the first fuel cell 10 tothe requested power amount when the requested power decreases to thefirst threshold.

For this reason, with the rates of increase in the power generationamounts of the first fuel cell 10 and the second fuel cell 11 beingtaken into account, the second threshold can be set at such a value thatswitching can be smoothly performed between power generation primarilyfrom the first fuel cell 10 and power generation primarily from thesecond fuel cell 11. In other words, the second threshold can be set atsuch a value that the power generation amount of the second fuel cell 11increases from almost 0 to the requested power amount when the requestedpower increases from the first threshold to the second threshold, andthe power generation amount of the first fuel cell 10 increases fromalmost 0 to the requested power amount when the requested powerdecreases from the second threshold to the first threshold.

In a case where the control unit 20 determines the requested power to beequal to or greater than the third threshold in step S22 (step S22: No),the control unit 20 causes both the first fuel cell 10 and the secondfuel cell 11 to generate electric power to provide the requested power(step S26). With this, after time 3 at which the requested power becomesequal to or greater than the third threshold, the requested power isprovided by both the power generation from the first fuel cell 10 andthe power generation from the second fuel cell 11, as shown in FIG. 6.Note that, in this case, the control unit 20 turns on the switches 86 aand 86 b, to electrically connect the first and second fuel cells 10 and11 to the motor generator 83.

The control unit 20 then determines whether there is an acceleratorposition signal acquired from the accelerator pedal sensor 67 (stepS28). In a case where there is an acquired accelerator position signal(step S28: Yes), the control unit 20 returns to step S12. In a casewhere any accelerator position signal is no longer being acquired (stepS28: No), the control unit 20 suspends the power generation from thefirst and second fuel cells 10 and 11 (step S30), and ends the powergeneration control process.

FIG. 7 is a chart for explaining power generation control in the firstembodiment. Note that FIG. 7 illustrates an example in which the maximumpower output of the first fuel cell 10 is 30%, and the maximum poweroutput of the second fuel cell 11 is 70%, where the sum of the maximumpower output of the first fuel cell 10 and the maximum power output ofthe second fuel cell 11 is 100% (the sum will be hereinafter alsoreferred to as the total maximum power). In the example case describedbelow, the first threshold is 100% of the maximum power output of thefirst fuel cell 10, or 30% of the total maximum power. The thirdthreshold has a value 50% greater than the sum of the maximum poweroutput of the first fuel cell 10 and the maximum power output of thesecond fuel cell 11. Accordingly, the third threshold is 70% of thetotal maximum power, which is 100% of the maximum power output of thesecond fuel cell 11. Meanwhile, the second threshold is 35% of the totalmaximum power.

As shown in FIG. 7, when the requested power is smaller than 30% of thetotal maximum power (or smaller than the first threshold), the requestedpower is provided by power generation from the first fuel cell 10. Whenthe requested power is not smaller than 35% of the total maximum powerbut is smaller than 70% of the total maximum power (or is not smallerthan the second threshold but is smaller than the third threshold), therequested power is provided by power generation from the second fuelcell 11. When the requested power is equal to or greater than 70% of thetotal maximum power (or equal to or greater than the third threshold),the requested power is provided by power generation from both the firstfuel cell 10 and the second fuel cell 11.

Although FIG. 7 shows an example case where the first threshold has avalue that is 100% of the maximum power output of the first fuel cell10, the first threshold does not necessarily have such a value, and mayhave some other value. Although the third threshold has a value that is70% of the total maximum power in the example case shown in FIG. 7, thethird threshold does not necessarily have such a value, and may havesome other value that is greater than 50% of the total maximum power.For example, the third threshold may have a greater value than 60% ofthe total maximum power, or may have a greater value than 65% of thetotal maximum power.

A fuel cell system that includes two fuel cells with the same maximumpower outputs is now described as a comparative example. FIG. 8 is achart for explaining power generation control in the comparativeexample. In FIG. 8, the sum of the maximum power outputs of the two fuelcells is 100% (the sum will be hereinafter also referred to as the totalmaximum power), as in FIG. 7. In the comparative example, the maximumpower outputs of the two fuel cells are the same, and accordingly, themaximum power output of each of the two fuel cells is 50% of the totalmaximum power. Therefore, in a case where the requested power is greaterthan 50% of the total maximum power, the requested power is provided bypower generation from both of the two fuel cells, as shown in FIG. 8.That is, the requested power can be provided by power generation of oneof the two fuel cells, only when the requested power is equal to orsmaller than 50% of the total maximum power.

In the first embodiment, on the other hand, the maximum power output ofthe second fuel cell 11 is greater than that of the first fuel cell 10.When the requested power is smaller than the first threshold, thecontrol unit 20 causes the first fuel cell 10 to generate electric powerso that the requested power is provided by the first fuel cell 10. Thatis, when the requested power is smaller than the first threshold, thecontrol unit 20 causes the first fuel cell 10 to generate electric powergreater than the electric power of the second fuel cell 11 so that therequested power is supplied. When the requested power is equal to orgreater than the second threshold, and is smaller than the thirdthreshold that is greater than the second threshold and is greater than50% of the sum of the maximum power output of the first fuel cell 10 andthe maximum power output of the second fuel cell 11, the second fuelcell 11 is made to generate electric power so that the requested poweris provided by the second fuel cell 11. That is, when the requestedpower is equal to or greater than the second threshold, and is smallerthan the third threshold that is greater than the second threshold andis greater than 50% of the sum of the maximum power output of the firstfuel cell 10 and the maximum power output of the second fuel cell 11,the second fuel cell 11 is made to generate electric power greater thanthe electric power of the first fuel cell 10 so that the requested poweris supplied. When the requested power is equal to or greater than thethird threshold, the first fuel cell 10 and the second fuel cell 11 aremade to generate electric power so that the requested power is providedby both the first fuel cell 10 and the second fuel cell 11. That is,when the requested power is equal to or greater than the thirdthreshold, both the first fuel cell 10 and the second fuel cell 11 aremade to generate electric power so that the requested power is supplied.Thus, the output range in which the requested power is provided by powergeneration from both the first fuel cell 10 and the second fuel cell 11can be made smaller than that in the comparative example that includestwo fuel cells with the same maximum power outputs, as shown in FIGS. 7and 8. As a result, the time during which the requested power isprovided by power generation from both the first fuel cell 10 and thesecond fuel cell 11 can be made shorter. Accordingly, the time duringwhich power generation from the first fuel cell 10 and/or the secondfuel cell 11 is suspended can be made longer. Thus, degradation due topotential variation during power generation from the first fuel cell 10and/or the second fuel cell 11 can be reduced, and durability of thefuel cell system 100 can be improved.

To extend the time during which only the second fuel cell 11 generateselectric power, the third threshold may have a value that is equal to orgreater than 70% of the total power output of the first fuel cell 10 andthe second fuel cell 11, may have a value that is equal to or greaterthan 80% of the total power output, may have a value that is equal to orgreater than 70% of the maximum power output of the second fuel cell 11,or may have a value that is equal to or greater than 80% of the maximumpower output of the second fuel cell 11. Alternatively, to preventdecrease in power generation efficiency and/or prevent rapid decrease involtage, the third threshold may have a value that is equal to orsmaller than 95% of the maximum power output of the second fuel cell 11,or may have a value that is equal to or smaller than 90% of the maximumpower output of the second fuel cell 11. Likewise, to extend the timeduring which only the first fuel cell 10 generates electric power, thefirst threshold may have a value that is equal to or greater than 70% ofthe maximum power output of the first fuel cell 10, or may have a valuethat is equal to or greater than 80% of the maximum power output of thefirst fuel cell 10. To prevent a decrease in power generation efficiencyand/or prevent a rapid decrease in voltage, the first threshold may havea value that is equal to or smaller than 95% of the maximum power outputof the first fuel cell 10, or may have a value that is equal to orsmaller than 90% of the maximum power output of the first fuel cell 10.

As shown in FIGS. 5 and 6, when the requested power is smaller than thefirst threshold, the control unit 20 suspends power generation from thesecond fuel cell 11. When the requested power is equal to or greaterthan the second threshold and is smaller than the third threshold, thecontrol unit 20 suspends power generation from the first fuel cell 10.In this manner, the time during which power generation from the firstfuel cell 10 and the second fuel cell 11 is suspended can be madelonger, and durability of the fuel cell system 100 can be improved. Notethat, the control unit 20 may suspend power generation from the secondfuel cell 11 when the requested power is smaller than the firstthreshold, and/or suspend power generation from the first fuel cell 10when the requested power is equal to or greater than the secondthreshold and is smaller than the third threshold.

As shown in FIG. 6, the second threshold has a greater value than thefirst threshold. When the requested power is equal to or greater thanthe first threshold and is smaller than the second threshold, thecontrol unit 20 causes the first fuel cell 10 and the second fuel cell11 to generate electric power so that the requested power is provided byboth the first fuel cell 10 and the second fuel cell 11. As a result,the output power of the second fuel cell 11 can quickly provide therequested power when the requested power increases to the secondthreshold, and the output power of the first fuel cell 10 can quicklyprovide the requested power when the requested power decreases to thefirst threshold. In this case, when the requested power is equal to orgreater than the first, threshold and is smaller than the secondthreshold, the control unit 20 preferably decreases the output power ofthe first fuel cell 10 and increases the output power of the second fuelcell 11 in response to an increase in the requested power, as shown inFIG. 6.

Second Embodiment

The configuration of a fuel cell system according to a second embodimentis the same as that of the first embodiment shown in FIG. 1, and theelectrical configuration thereof is the same as that of the firstembodiment shown in FIG. 4. Therefore, those configurations are notexplained herein. FIG. 9 is a flowchart showing a power generationcontrol process in the second embodiment. FIG. 10 is a timing chart forexplaining power generation control in the second embodiment. As shownin FIG. 9, the control unit 20 first carries out steps S40 through S44.Steps S40 through S44 are the same as steps S10 through S14 of the firstembodiment shown in FIG. 5, and therefore, are not explained herein.

In a case where the control unit 20 determines the requested power to besmaller than the first threshold in step S44 (step S44: Yes), thecontrol unit 20 causes the first fuel cell 10 to generate electric powerso that the requested power is provided by the first fuel cell 10, andsuspends the power generation from the second fuel cell 11 (step S46).As a result, the requested power is provided by the power generationfrom the first fuel cell 10 until time 1, before which the requestedpower is smaller than the first threshold, as shown in FIG. 10.

In a case where the control unit 20 determines the requested power notto be smaller than the first threshold in step S44 (step S44: No), thecontrol unit 20 then determines whether the requested power is equal toor greater than the first threshold and is smaller than the thirdthreshold (step S48). In a case where the control unit 20 determinesthat the requested power is equal to or greater than the first thresholdand is smaller than the third threshold in step S48 (step S48: Yes), thecontrol unit 20 suspends the power generation from the first fuel cell10, and causes the second fuel cell 11 to generate electric power sothat the requested power is provided by the second fuel cell 11 (stepS50). As a result, during the period from time 1 to time 3, during whichthe requested power is equal to or greater than the first threshold andis smaller than the third threshold, the requested power is provided bythe power generation from the second fuel cell 11, as shown in FIG. 10.

In a case where the control unit 20 determines that the requested poweris equal to or greater than the third threshold in step S48 (step S48:No), the control unit 20 causes both the first fuel cell 10 and thesecond fuel cell 11 to generate electric power to provide the requestedpower (step S52). As a result, after time 3 at which the requested powerbecomes equal to or greater than the third threshold, the requestedpower is provided by the power generation from both the first fuel cell10 and the second fuel cell 11, as shown in FIG. 10.

The control unit 20 then determines whether there is an acceleratorposition signal acquired from the accelerator pedal sensor 67 (stepS54). In a case where there is an acquired accelerator position signal(step S54: Yes), the control unit 20 returns to step S42. In a casewhere any accelerator position signal is no longer being acquired (stepS54: No), on the other hand, the control unit 20 suspends the powergeneration from the first and second fuel cells 10 and 11 (step S56),and ends the power generation control process.

In the first embodiment, in a case where the requested power is equal toor greater than the first threshold and is smaller than the secondthreshold that is greater than the first threshold, both the first fuelcell 10 and the second fuel cell 11 are made to generate electric power,so that the output power of the first fuel cell 10 and the second fuelcell 11 quickly provides the requested power. However, as describedabove, even when the reaction gases are being supplied to the first fuelcell 10, it is possible to suspend the power generation from the firstfuel cell 10 by turning off the switch 86 a. Even when the reactiongases are being supplied to the second fuel cell 11, it is possible tosuspend the power generation from the second fuel cell 11 by turning offthe switch 86 b. Accordingly, it is possible to rapidly increase theoutput power of the first fuel cell 10 and the second fuel cell 11 toquickly provide the requested power, by controlling the reaction gasesto be supplied to the first fuel cell 10 and the second fuel cell 11 andcontrolling the switching on and off of the switches 86 a and 86 b.Therefore, the second threshold may not be set (in other words, thesecond threshold may be set at the same value as the first threshold).

FIG. 11 is a chart for explaining power generation control in the secondembodiment. In FIG. 11, the sum of the maximum power output of the firstfuel cell 10 and the maximum power output of the second fuel cell 11 is100%, the first threshold is 30% of the total maximum power, and thethird threshold is 70% of the total maximum power, as in FIG. 7. Asshown in FIG. 11, when the requested power is smaller than 30% of thetotal maximum power (or smaller than the first threshold), the firstfuel cell 10 is made to generate electric power to provide the requestedpower. When the requested power is equal to or greater than 30% and issmaller than 70% (or is equal to or greater than the first threshold andis smaller than the third threshold), the second fuel cell 11 is made togenerate electric power to provide the requested power. When therequested power is equal to or greater than 70% of the total maximumpower (or is equal to or greater than the third threshold value), boththe first fuel cell 10 and the second fuel cell 11 are made to generateelectric power to provide the requested power. As the second thresholdvalue is not set (in other words, the second threshold is set at thesame value as the first threshold) as described above, both the timeduring which only the first fuel cell 10 generates electric power andthe time during which only the second fuel cell 11 generates electricpower or one of the time during which only the first fuel cell 10generates electric power and the time during which only the second fuelcell 11 can be made longer than that in the first embodiment. As aresult, the time during which power generation from the first fuel cell10 and/or the second fuel cell 11 is suspended can be made longer thanthat in the first embodiment. Thus, durability of the fuel cell systemcan be further improved.

Third Embodiment

The configuration of a fuel cell system according to a third embodimentis the same as that of the first embodiment shown in FIG. 1, and theelectrical configuration thereof is the same as that of the firstembodiment shown in FIG. 4. Therefore, those configurations are notexplained herein. FIG. 12 is a flowchart showing a power generationcontrol process in the third embodiment. FIG. 13 is a timing chart forexplaining power generation control in the third embodiment. As shown inFIG. 12, the control unit 20 first carries out steps S60 through S64.Steps S60 through S64 are the same as steps S10 through S14 of the firstembodiment shown in FIG. 5, and therefore, are not explained herein.

In a case where the control unit 20 determines the requested power to besmaller than the first threshold in step S64 (step S64: Yes), thecontrol unit 20 causes the first fuel cell 10 to generate electric powerso that the requested power is provided by the first fuel cell 10, andsuspends the power generation from the second fuel cell 11 (step S66).As a result, the requested power is provided by the power generationfrom the first fuel cell 10 until time 1, before which the requestedpower is smaller than the first threshold, as shown in FIG. 13.

In a case where the control unit 20 determines the requested power notto be smaller than the first threshold in step S64 (step S64: No), thecontrol unit 20 then determines whether the requested power is equal toor greater than the first threshold and is smaller than the secondthreshold (step S68). In a case where the control unit 20 determinesthat the requested power is equal to or greater than the first thresholdand is smaller than the second threshold (step S68: Yes), the controlunit 20 causes both the first fuel cell 10 and the second fuel cell 11to generate electric power to provide the requested power (step S70). Asa result, during the period from time 1 to time 2, during which therequested power is equal to or greater than the first threshold and issmaller than the second threshold, the requested power is provided bythe power generation from both the first furl cell 10 and the secondfuel cell 11, as shown in FIG. 13.

In a case where the control unit 20 determines the requested power notto be equal to or greater than the first threshold and not to be smallerthan the second threshold in step S68 (step S68: No), the control unit20 determines whether the requested power is equal to or greater thanthe second threshold and is smaller than the third threshold (step S72).In a case where the control unit 20 determines that the requested poweris equal to or greater than the second threshold and is smaller than thethird threshold (step S72: Yes), the control unit 20 causes the firstfuel cell 10 to generate electric power on low power while maintainingsuch a potential range that fuel cell degradation can be prevented, andcauses the second fuel cell 11 to generate electric power equivalent tothe difference between the requested power and the electric power beinggenerated from the first fuel cell 10 (step S74). Here, a potential thatcan prevent fuel cell degradation is equivalent to a state in which thechange in potential is small when the voltage per unit cell in the fuelcell is in the range of 0.7 V to 0.9 V. Note that the potential may beequivalent to a state in which the change in potential is small when thevoltage per unit cell in the fuel cell is in the range of 0.7 V to 0.8 Vor a state in which the change in potential is small when the voltageper unit cell in the fuel cell is in the range of 0.75 V to 0.8 V. Thecontrol unit 20 drives the air compressor 32, the injector 54, and thelike, so that the necessary amounts of air and hydrogen for the abovepower generation are supplied to the first fuel cell 10. While doing so,the control unit 20 controls the FDC 81 a, so that electric power isgenerated at a substantially constant voltage to maintain the voltageper unit cell in the first fuel cell 10 in the range of 0.76 V to 0.78V, for example. The reason that the voltage per unit cell is maintainedin a predetermined range will be described later. The control unit 20also drives the air compressor 42, the injector 74, and the like, sothat the necessary amounts of air and hydrogen for generating an amountof electric power equivalent to the difference between the requestedpower and the electric power being generated from the first fuel cell 10are supplied to the second fuel cell 11. As a result, during the periodfrom time 2 to time 3, during which the requested power is equal to orgreater than the second threshold and is smaller than the thirdthreshold, as shown in FIG. 13, the requested power is providedprimarily by the second fuel cell 11, and the first fuel cell 10generates electric power at a constant voltage. Here, the requestedpower is provided primarily by the second fuel cell 11 in a case wherethe first fuel cell 10 generates electric power at a constant voltage,and the second fuel cell 11 generates electric power equivalent to thedifference between the requested power and the electric power beinggenerated from the first fuel cell 10, as described above. For example,the requested power is provided primarily by the second fuel cell 11 ina case where the output power of the second fuel cell 11 is equal to orgreater than 85% of the requested power, where the output power of thesecond fuel cell 11 is equal to or greater than 90% of the requestedpower, or where the output power of the second fuel cell 11 is equal toor greater than 95% of the requested power.

In a case where the control unit 20 determines that the requested poweris equal to or greater than the third threshold in step S72 (step S72:No), the control unit 20 causes both the first fuel cell 10 and thesecond fuel cell 11 to generate electric power to provide the requestedpower (step S76). As a result, after time 3, at which the requestedpower becomes equal to or greater than the third threshold, therequested power is provided by the power generation from both the firstfuel cell 10 and the second fuel cell 11, as shown in FIG. 13.

The control unit 20 then determines whether there is an acceleratorposition signal acquired from the accelerator pedal sensor 67 (stepS78). In a case where there is an acquired accelerator position signal(step S78: Yes), the control unit 20 returns to step S62. In a casewhere any accelerator position signal is no longer being acquired (stepS78: No), the control unit 20 suspends the power generation from thefirst and second fuel cells 10 and 11 (step S80), and ends the powergeneration control process.

In a fuel cell, an oxide coating is formed on a catalyst metal surfaceat high potential, and the oxide coating is removed at low potential.When the potential becomes high with the oxide coating of the catalystremoved, the catalyst is easily eluted, resulting in decrease in powergenerating performance. For example, when the requested power is equalto or greater than the second threshold and is smaller than the thirdthreshold, and power generation from the first fuel cell 10 iscompletely stopped, the potential of the first fuel cell 10 becomes toohigh, and the power generating performance might be degraded due toelution of the catalyst. To counter this, when the requested power isequal to or greater than the second threshold and is smaller than thethird threshold, the control unit 20 in the third embodiment causes thefirst fuel cell 10 to generate electric power at a constant voltage insuch a range that elution of the catalyst contained in the first fuelcell 10 is inhibited, and causes the second fuel cell 11 to generateelectric power equivalent to the difference between the requested powerand the electric power being generated from the first fuel cell 10, sothat the requested power is provided primarily by the second fuel cell11. In this manner, the potential of the first fuel cell 10 can beprevented from become too high, and decrease in the power generatingperformance of the first fuel cell 10 can be prevented.

In the third embodiment, when the requested power is equal to or greaterthan the second threshold and is smaller than the third threshold, thefirst fuel cell 10 is made to generate electric power at a constantvoltage. Instead of or in addition to that, when the requested power issmaller than the first threshold, the second fuel cell 11 may be made togenerate electric power at a constant voltage in such a range thatelution of the catalyst contained in the second fuel cell 11 isinhibited, and the first fuel cell 10 may be made to generate electricpower equivalent to the difference between the requested power and theelectric power being generated from the second fuel cell 11, so that therequested power can be provided primarily by the first fuel cell 10. INthis manner, the potential of the second fuel cell 11 can be preventedfrom becoming too high, and decrease in the power generating performanceof the second fuel cell 11 can be prevented. Here, the requested poweris provided primarily by the first fuel cell 10 in a case where thesecond fuel cell 11 generates electric power at a constant voltage, andthe first fuel cell 10 generates electric power equivalent to thedifference between the requested power and the electric power beinggenerated from the second fuel cell 11. For example, the requested poweris provided primarily by the first fuel cell 10 in a case where theoutput power of the first fuel cell 10 is equal to or greater than 85%of the requested power, is equal to or greater than 90% of the requestedpower, or is equal to or greater than 95% of the requested power.

In the first through third embodiments, the first threshold, the secondthreshold, and the third threshold may be fixed values that are notallowed to be changed, or may be variable values that can be changed. Ina case where the rate of change in the requested power is equal to orhigher than a predetermined value, the first threshold and/or the thirdthreshold may be made smaller than in a case where the rate of change inthe requested power is lower than the predetermined value. As a result,the requested power can be provided by power generation from the firstfuel cell 10 and the second fuel cell 11 even in a case where therequested power rapidly changes. Further, in a case where a power mode(power performance priority control) and an ecological mode (fuelefficiency priority control) are set, and the power mode is selected,the control unit 20 may make the first threshold and/or the thirdthreshold smaller than in a case where the ecological mode is selected.

Fourth Embodiment

FIG. 14 is a schematic diagram showing the configuration of a fuel cellsystem according to a fourth embodiment. As shown in FIG. 14, in a fuelcell system 400 according to the fourth embodiment, the control unit 20includes a storage unit 24 and a communication unit 26, as well as thepower generation controller 22. The storage unit 24 stores an operationhistory of the vehicle equipped with the fuel cell system 400, forexample. FIG. 15 is a chart showing an example of the operation historystored in the storage unit 24. As shown in FIG. 15, the operationhistory stored in the storage unit 24 is a map showing the correlationbetween the requested power and operation times of the first fuel cell10 and the second fuel cell 11. In FIG. 15, the first fuel cell 10 isprimarily operated in a region 91 where the requested power is smallerthan the first threshold, and the second fuel cell 11 is primarilyoperated in a region 92 where the requested power is equal to or greaterthan the second threshold and is smaller than the third threshold. Thecommunication unit 26 will be described later.

On the basis of the operation history stored in the storage unit 24, thecontrol unit 20 may change the first threshold and the third thresholdso that the first fuel cell 10 and the second fuel cell 11 havesubstantially the same degree of degradation. For example, based on theoperation history that is a map stored in the storage unit and showingthe correlation between the requested power and the operation time asshown in FIG. 15, the control unit 20 may change the first thresholdand/or the third threshold so that the operation time of the first fuelcell 10 (or the area of the region 91) becomes substantially the same asthe operation time of the second fuel cell 11 (or the area of the region92). As a result, the degrees of degradation of the first fuel cell 10and the second fuel cell 11 can be made substantially the same inaccordance with the operation pattern of the vehicle (such as a casewhere the vehicle is driven mostly in urban areas, or a case where thevehicle is driven mostly on expressways). Thus, durability of the fuelcell system 400 can be further improved. Where the operation time of thefirst fuel cell 10 is substantially the same as the operation time ofthe second fuel cell 11, the area of the region 92 representing theoperation time of the second fuel cell 11 may be equal to or larger than80% and be equal to or smaller than 120% of the area of the region 91representing the operation time of the first fuel cell 10, or may beequal to or larger than 90% and be equal to or smaller than 110% of thearea of the region 91. Note that the operation history is notnecessarily a map showing the relationship between the requested powerand the operation time. The operation history may be a map showing therelationship between the requested power and the total power generationamount, or a map showing the requested power and the number of times therequested power is exceeded, or a combination of these maps.

The communication unit 26 exchanges data with an external server 90. Forexample, the communication unit 26 transmits an operation history (suchas a map showing the correlation between requested power and operationtime as shown in FIG. 15, for example) to the external server 90. Thecommunication unit 26 also receives, from the external server 90, anoperation history (such as a map showing the correlation betweenrequested power and operation time as shown in FIG. 15, for example)that has been transmitted from another vehicle and been stored in theexternal server 90.

On the basis of an operation history of the fuel cell system of anothervehicle received from the external server 90, the control unit 20 maychange the first threshold and the third threshold so that the firstfuel cell 10 and the second fuel cell 11 have substantially the samedegree of degradation. For example, on a basis of an operation historyreceived from the external server 90 and formed with a map showing thecorrelation between requested power and operation time as shown in FIG.15, the first threshold and/or the third threshold may be changed sothat the operation time of the first fuel cell 10 and the operation timeof the second fuel cell 11 become substantially the same. As a result,even in a case where the fuel cell system 400 has been operated only fora short time, the degree of degradation of the first fuel cell 10 andthe degree of degradation of the second fuel cell 11 can be madesubstantially the same, and durability of the fuel cell system 400 canbe improved. In a case where there are areas set for respective cities,for example, the control unit 20 may acquire an operation history fromthe external server 90 located in the area corresponding to the currentposition of the vehicle equipped with the fuel cell system 400 or theposition in which the vehicle is normally stored.

In the example cases described in the first through fourth embodiments,two fuel cells are included in a fuel cell system. However, three ormore fuel cells may be included in a fuel cell system. In that case, twoof the three or more fuel cells are equivalent to the first fuel cell 10and the second fuel cell 11 described in the first through fourthembodiments.

Although some embodiments of the present disclosure have been describedin detail, the present disclosure is not limited to the specificembodiments but may be varied or changed within the scope of the presentdisclosure as claimed.

What is claimed is:
 1. A fuel cell system comprising: a first fuel cell;a second fuel cell having a maximum power output that is greater than amaximum power output of the first fuel cell; and a power generationcontroller configured to control power generation from the first fuelcell and the second fuel cell in accordance with requested power,wherein, when the requested power is smaller than a first threshold, thepower generation controller is configured to cause the first fuel cellto generate electric power greater than electric power of the secondfuel cell so that the requested power is supplied, when the requestedpower is equal to or greater than a second threshold that is equal to orgreater than the first threshold, and is smaller than a third thresholdthat is greater than the second threshold and is greater than 50% of asum of the maximum power output of the first fuel cell and the maximumpower output of the second fuel cell, the power generation controller isconfigured to cause the second fuel cell to generate electric powergreater than electric power of the first fuel cell so that the requestedpower is supplied, and when the requested power is equal to or greaterthan the third threshold, the power generation controller is configuredto cause both the first fuel cell and the second fuel cell to generateelectric power so that the requested power is supplied.
 2. The fuel cellsystem according to claim 1, wherein the power generation controller isconfigured to suspend power generation from the second fuel cell whenthe requested power is smaller than the first threshold, and/or isconfigured to suspend power generation from the first fuel cell when therequested power is equal to or greater than the second threshold and issmaller than the third threshold.
 3. The fuel cell system according toclaim 1, wherein the power generation controller is configured to causethe second fuel cell to generate electric power at a voltage at whichelution of a catalyst contained in the second fuel cell is inhibitedwhen the requested power is smaller than the first threshold, and/or isconfigured to cause the first fuel cell to generate electric power at avoltage at which elution of a catalyst contained in the first fuel cellis inhibited when the requested power is equal to or greater than thesecond threshold and is smaller than the third threshold.
 4. The fuelcell system according to claim 1, wherein the second threshold has agreater value than the first threshold, and, when the requested power isequal to or greater than the first threshold and is smaller than thesecond threshold, the power generation controller is configured to causethe first fuel cell and the second fuel cell to generate electric powerso that the requested power is provided by both the first fuel cell andthe second fuel cell.
 5. The fuel cell system according to claim 4,wherein, when the requested power is equal to or greater than the firstthreshold and is smaller than the second threshold, the power generationcontroller is configured to cause output power of the first fuel cell tobecome smaller and is configured to cause output power of the secondfuel cell to become larger in response to an increase in the requestedpower.
 6. The fuel cell system according to claim 1, wherein the secondthreshold has the same value as the first threshold.
 7. The fuel cellsystem according to claim 1, wherein, when a rate of change in therequested power is equal to or higher than a predetermined value, thepower generation controller is configured to reduce at least one of thefirst threshold and the third threshold to a smaller value than in acase where the rate of change in the requested power is lower than thepredetermined value.
 8. The fuel cell system according to claim 1,further comprising a storage unit that stores a map showing acorrelation between requested power and an operation time of each of thefirst fuel cell and the second fuel cell, wherein, on the basis of themap stored in the storage unit, the power generation controller isconfigured to change at least one of the first threshold and the thirdthreshold so that the operation time of the second fuel cell becomesequal to or greater than 80% of the operation time of the first fuelcell and equal to or less than 120% of the operation time of the firstfuel cell.
 9. The fuel cell system according to claim 1, wherein, on thebasis of a map received from an external server and showing acorrelation between requested power and an operation time in anotherfuel cell system, the power generation controller is configured tochange at least one of the first threshold and the third threshold sothat an operation time of the second fuel cell becomes equal to orgreater than 80% of an operation time of the first fuel cell and equalto or less than 120% of the operation time of the first fuel cell.